Owing to such physical and chemical properties as outstanding heat resistance and mechanical strength, as well as strength against radiation, silicon carbide (SiC) has attracted attention as an environmentally rugged semiconductor material. However, SiC is a typical substance having a polytype structure that assumes numerous different crystal structures even though the chemical composition is the same. Where the molecules of bonded Si and C in the crystal structure are thought of as units, the polytypes occur because the periodic structure varies when these molecular structure units stack in the C-axis direction ([0001] direction of the crystal. The typical polytypes here are 6H, 4H, 15R and 3C, where the initial numeral indicates the stacking sequence period and the alphabetic character represents the crystal system (H for hexagonal system, R for rhombohedral system, and C for cubic system). Moreover, the individual polytypes differ in physical and electrical characteristics, and various applications are devised using these differences.
Single-crystal SiC of a size enabling fabrication of semiconductor devices has up to now been obtained on a laboratory scale by, for example, using the sublimation recrystallization method (Lely process) to grow single-crystal SiC. However, the area of the single crystal obtained by this method is small and its size and shape are hard to control with high accuracy. Further, the polytype and impurity carrier density possessed by the SiC is also not easy to control.
Further, growth of cubic single-crystal SiC is conducted by using the chemical vapor deposition method (CVD method) to heteroepitaxially grow silicon (Si) or the like on a foreign substrate. However, while large-area single crystal can be obtained by this method, high-quality single-crystal SiC cannot be readily obtained because only single-crystal SiC containing many defects (up to 107 cm−2) can be grown since, inter alia, the lattice mismatch with the substrate is as large as around 20%.
Therefore, in order to resolve these difficulties, the modified Lely process, which conducts sublimation recrystallization using a single-crystal SiC [0001] oriented substrate, was proposed (Non-patent Document 1). This sublimation recrystallization method is a method of producing SiC crystal by sublimating SiC powder at a high temperature above 2000° C. and recrystallizing the sublimation gas in a low-temperature region, which is called the modified Lely process and used especially for producing bulk single-crystal SiC. And since a seed crystal is used in the modified Lely process, the crystal nucleation process can be controlled, and by using an inert gas to control the ambient pressure to about 100 Pa to 15 kPa, the crystal growth rate and the like can be controlled with good reproducibility.
Here, by way of explaining the modified Lely process, single-crystal SiC constituting a seed crystal whose crystal growth surface is the [0001] plane and single-crystal SiC powder constituting the raw material (one obtained by washing and preprocessing an abrasive produced by the Acheson method is ordinarily used) are, as shown in FIG. 8, placed in a crucible (although usually made of graphite, a material other than graphite, such as a high-melting-point material, graphite-coated high-melting-point material, or graphite coated with high-melting-point material, is sometimes used partially) and heated to 2000 to 2400° C. in an argon or other inert gas atmosphere (133 Pa to 13.3 kPa). At this time, the temperature gradient is established so that the seed crystal is somewhat lower in temperature than the raw material powder (e.g., 100 to 200° C. lower). After sublimation, the raw material is diffused and conveyed toward the seed crystal by the density gradient (formed by the temperature gradient). Single crystal growth is realized by recrystallization on the seed crystal of the raw material gas arriving at the seed crystal. At this time, addition of impurity gas into atmosphere consisting of inert gas or mixing of an impurity element or a compound thereof with the SiC raw material powder enables the volume resistivity of the crystal to control the substitution (doping) of an impurity element at the position of a silicon or carbon atom in the single crystal SiC structure. Among the typical substitutional impurities in the single-crystal SiC here are nitrogen (n 25 type), boron, and aluminum (p type). Single-crystal SiC can be grown while using these impurities to control the carrier type and density. Single-crystal SiC wafers of 2-inch (50.8 mm) to 4-inch (100 mm) diameter are currently being cut from single-crystal SiC produced by the aforesaid Lely process and utilized for epitaxial film growth and device fabrication.
As set out above, in the modified Lely process, raw material sublimated at high temperature in a graphite crucible is diffused by a density gradient formed by a temperature gradient and conveyed onto a seed crystal where it is recrystallized to grow single-crystal SiC. During crystal growth, therefore, the effect exerted on crystal growth by the flow of sublimation gas in the crucible is large. So, with the aim of controlling the gas flow rate so as to collect the sublimation gas generated from the raw material at the seed crystal substrate, cases have been reported, in Patent Document 1 for example, of providing a conical flange at the seed crystal attachment region.